1. Field of the Invention
The present invention relates to a clipping circuit and a radio transmitter using the same, and in particular, to a clipping method for use in a radio transmitter for limiting the amplitude of orthogonal baseband signals supplied to a power amplifier in a radio transmitter.
2. Description of the Prior Art
In a radio communication system, it is ideal on the side of a radio transmitter to use a power amplifier having an appropriate linear characteristic for all amplitude values of transmission signals. However, such a power amplifier with the favorable linearity for all transmissible amplitude values cannot be easily implemented in consideration of a circuit size, cost of the amplifier circuit, and power consumed by the circuit. There is hence employed in many cases a general power amplifier having linearity up to a particular amplitude and nonlinearity beyond the amplitude.
When a power amplifier circuit amplifies a transmission signal multiplexed for a plurality of transmission channels using, for example, a Code Division Multiple Access (CDMA) scheme or an Orthogonal Frequency Division Multiplexing (OFDM) scheme, the width of the amplitude value of a signal to be amplified becomes greater as the number of channels of signals to be multiplexed into one Radio Frequency (RF; high frequency) output signal increases. When this signal is amplified by the inappropriate power amplifier described above, the RF output signal is distorted if the signal has an amplitude value exceeding an amplitude value range associated with the linear characteristic of the power amplifier. The output signal distorted in this manner interferes adjacent communication channels and deteriorates modulation accuracy, resulting in a deteriorated Bit Error Rate (BER) of a transmission path employed to send the signal.
Various proposals have been offered to suppress the distortion of the RF output signal. An example is the clipping of a baseband signal including I and Q signals at a baseband signal processing module to process the transmission data. Representative clipping methods include a rectangular clipping scheme described in Japanese Patent Application Laid-Open No. 63-198174 and a circular clipping scheme.
Although the rectangular clipping is feasible by a small-sized circuit, a phase error disadvantageously takes place in the clipped data. In the rectangular clipping method, I dicrection clipping is independently performed for an I component of the baseband signal and Q direction clipping is also carried out independently for a Q component of the baseband signal. Therefore, if the I component exceeds a clip level indicated by a broken line and the Q component is below the clip level, as shown in FIG. 10A, only the I component is clipped. In a vector element obtained as a result of the clipping, there appears a phase error of θ with respect to the original vector element. Occurrence of the phase error deteriorates the Error Vector Magnitude (EVM) of the modulated wave.
On the other hand, in the circular clipping scheme, the I and Q components of the baseband signal are clipped on the basis of the phase. This does not cause the phase error of the rectangular clipping (FIG. 10B). However, since the circular clipping is conducted through processing of arithmetic calculation or by reading data items from a Read Only Memory (ROM) table, there exists a disadvantage that the circuit size becomes larger as the number of bits of the I and Q components increases. It is also disadvantageous that the circuit size considerably varies depending on an input dynamic range assumed for the circuit operation.